This invention relates to a drive system for a vehicle.
Certain current production tractors include infinitely variable transmissions which provide an infinitely variable ratio between the engine speed and the driven wheel speed. When such a tractor is pulling a heavy trailer and is on a down slope or is decelerating, the load from the trailer must be countered by traction forces from the tires. Traction forces result in wheel slip, and large values of wheel slip reduce the ability of the tires to provide directional control or steering forces. As wheel slip increases, the tire loses its ability to provide steering force and thus the tractor loses directional stability. Thus, such an overrunning load can cause loss of directional stability.
To prevent this loss of directional stability on some current production tractors with IVT's, the rear wheel slip is limited by increasing the rear wheel speed when excessive slip is detected.
To accomplish this, the rotational speed of the front axle is sensed and this sensed speed is used as to compute a vehicle speed estimate. The front wheels are free wheeling when pulling a trailer at highway speeds, and therefore, front tire slip is negligible, and the front axle speed is a good indication of vehicle speed. Similarly, a second vehicle speed is estimated from the rear axle speed. This second speed estimated is affected by wheel slip resulting from the generation of traction forces. The rear wheel slip is calculated by computing the ratio of the speed estimates based on the rear and front wheel speeds. Because of the geometry of the tractor axles in a conventionally steered, rear wheel driven tractor, the front wheel speed increases when turning, therefore, the ratio of the rear axle to front axle drive speed is decreased when turning, and if the front axle speed is used to estimate true vehicle speed in calculating tire slip with an overrunning load, the calculated slip would be excessive and the wheel slip control algorithm would overcorrect.
Vehicle electric drive systems or AC electric traction drives have been proposed, as described in U.S. Pat. No. 5,568,023 issued 22 Oct. 1996 and in U.S. Pat. No. 6,492,785, issued 10 Dec. 2002. Such an electric drive system typically includes an engine-driven 3-phase electric motor/generator coupled to an inverter/rectifier, which, in turn, is coupled to a DC buss. The buss feeds an inverter/rectifier which supplies power to a traction motor/generator which drives an axle or a wheel. The inverter/rectifiers invert the DC current on the buss to 3-phase AC current at a frequency to drive the wheels at the speed directed by the operator. It would be desirable to apply a steering responsive slip limiting function to such a vehicle electric drive system.